1. Field of the Invention
The present invention relates to the manufacture of circuit boards. More particularly, the invention relates to a continuous process for forming a multilayered circuit structure, which prevents damage to conductive foils during the formation of multilayered circuit structures while enhancing etching precision and accuracy of the circuits.
2. Description of the Related Art
Circuit boards and printed circuits have wide application in the field of electronics. They are useful for large scale applications, such as in industrial control equipment, as well as in small scale devices, such as telephones, radios and personal computers. In producing such printed circuits, it is important that a high degree of accuracy and resolution is attained for very small line and space widths to ensure good performance of the circuit.
The ability to produce accurate features having very small dimensions, particularly of 100xcexc or less, is extremely important in the production of small and large scale equipment. Etching precision becomes more important as the circuit patterns become ever smaller. It is well known in the art to use known photolithographic techniques to produce printed circuit boards having small features with a high degree of accuracy. Typically, an electrically conductive foil is deposited onto a substrate, and a photoresist is then deposited onto the foil. The photoresist is then imagewise exposed and developed, forming a pattern of small lines and spaces which are then etched into the conductive foil.
It is a common practice to then subject the foil to a bond enhancement such as a xe2x80x9cblack oxidexe2x80x9d treatment, in which the copper is pre-roughened by chemical micro-etching, and is chemically treated with a layer of copper oxide (which is black). This treatment helps to promote secure adhesion of the foil to other materials. See, for example, the discussion in U.S. Pat. No. 4,997,516, which is incorporated herein by reference, for a discussion of forming a black oxide on the surface of a foil. The adhesiveness of the foil surface to prepregs or other materials is greatly enhanced by the black oxide treatment, resulting in greater heat and moisture resistance of the resulting multilayer circuit structure.
One problem that arises in the formation of circuit structures is that damage to metallic foil surfaces, resin spots on the foil, and handling of thin laminates have been known to cause low yield. This damage is mainly caused by excessive manipulation of the foils during manual handling processes, such as those presently widely used in the art. It would therefore be desirable to employ a process for forming multilayer circuit structures which avoids or reduces damage and imperfections to metal foils, while etching circuit lines and spaces with high resolution and accuracy. The present invention provides a solution to this problem by providing a continuous process which minimizes manual handling of copper foils, to thereby avoid or reduce unnecessary damage to the foils.
Continuous processes are used in the manufacturing of flexible printed circuits that minimize yield losses due to material damage. The flexible substrate (usually a copper clad polyimide or polyester film) has a circuit pattern put on one or both sides. Typical process flows are described by J. Fjelstad, Flexible Circuit Technology, Silicon Valley Publishers Group, 1994. The reel to reel technique was also described by D. Weiss, et al xe2x80x9cManufacture of 4 Layer MCM-L""s Using Reel to Reel Manufacturing Methodsxe2x80x9d, Institute for Interconnecting and Packaging Electronic Circuits, 1997, in the production of four layer multichip modules made from epoxy laminate. He contended that if the epoxy substrate is extremely thin, it would be flexible enough to be processed in a continuous process. The problem with epoxy substrates is that the core thickness is limited to approximately 150 microns, since above that the substrate loses flexibility. Additionally the current flexible printed circuit process is limited in substrate thickness between 50 and 200 microns in 50 micron increments. The present invention addresses these issues in that a wide range of finished product thickness can be obtained in very small increments.
According to the present invention, multilayer circuit structures are formed by a continuous process which includes applying and curing a film forming polymer onto the matte side of a copper foil. The opposite (shiny) side of the foil is optionally but preferably cleaned, and applied with a photoresist which is then optionally but preferably dried. The photoresist is exposed, and developed to remove the nonimage areas but leave the image areas. The foil under the removed nonimage area is then etched to form a copper pattern, and the remaining photoresist is optionally but preferably removed. The foil is then cut into sections, and then optionally but preferably punched with registration holes. The copper pattern is then optionally but preferably treated with a bond enhancing treatment, optionally but preferably inspected for defects, and laminated onto a substrate to form a multilayered circuit structure. This approach is preferably conducted in a reel to reel manner. This technique results in more accurate etching and better etch uniformity than known methods, because the optimal orientation (face down) through the wet processing steps can be utilized.
The invention provides a continuous process for forming a multilayered circuit structure which comprises:
(a) unrolling a roll of copper foil, which foil has a shiny surface side and a matte surface side;
(b) applying and curing a film-forming polymer onto the matte side of the foil,
(c) optionally cleaning the shiny side of the foil;
(d) applying and optionally drying a photoresist onto the shiny side of the foil;
(e) imagewise exposing the photoresist to actinic radiation to thereby form image and nonimage areas;
(f) developing the photoresist thereby removing the nonimage areas and leaving the image areas;
(g) etching the foil under the removed nonimage areas of the photoresist to thereby form a copper pattern;
(h) optionally removing the remaining photoresist;
(i) cutting the foil into sections;
(j) optionally punching registration holes through the foil;
(k) optionally treating the copper pattern with a bond enhancing treatment;
(l) optionally inspecting the copper pattern for defects; and
(m) laminating at least one foil section to a substrate; thus forming a multilayered circuit structure.
The invention also provides a continuous process for forming a multilayered circuit structure which comprises:
(a) unrolling a roll of copper foil, which foil has a shiny surface side and a matte surface side;
(b) applying and curing a film-forming polymer onto the matte side of the foil,
(c) cleaning the shiny side of the foil;
(d) applying and drying a photoresist onto the shiny side of the foil;
(e) imagewise exposing the photoresist to actinic radiation to thereby form image and nonimage areas;
(f) developing the photoresist thereby removing the nonimage areas and leaving the image areas;
(g) etching the foil under the removed nonimage areas of the photoresist to thereby form a copper pattern;
(h) removing the remaining photoresist;
(i) cutting the foil into sections;
(j) punching registration holes through the foil;
(k) treating the copper pattern with a bond enhancing treatment;
(l) inspecting the copper pattern for defects; and
(m) laminating at least one foil section to a substrate; thus forming a multilayered circuit structure.
The invention further provides a continuous process for forming a multilayered circuit structure which comprises:
(a) unrolling a roll of copper foil, which foil has a shiny surface side and a matte surface side, both sides having been treated with bond enhancing treatment;
(b) applying and curing a film-forming polymer onto either side of the foil,
(c) optionally cleaning the side of the foil that has not been applied with the film-forming polymer;
(d) applying and optionally drying a photoresist onto the uncoated side of the foil;
(e) imagewise exposing the photoresist to actinic radiation to thereby form image and nonimage areas;
(f) developing the photoresist thereby removing the nonimage areas and leaving the image areas;
(g) etching the foil under the removed nonimage areas of the photoresist to thereby form a copper pattern;
(h) optionally removing the remaining photoresist;
(i) cutting the foil into sections;
(j) optionally punching registration holes through the foil;
(k) optionally inspecting the copper pattern for defects; and
(l) laminating at least one foil section to a substrate; thus forming a multilayered circuit structure.
The invention still further provides a continuous process for forming a multilayered circuit structure which comprises:
(a) unrolling a roll of copper foil, which foil has a shiny surface side and a matte surface side, whose shiny side has been treated with a bond enhancing treatment;
(b) applying and curing a film-forming polymer onto the shiny side of the foil,
(c) optionally cleaning the matte side of the foil;
(d) applying and optionally drying a photoresist onto the shiny side of the foil;
(e) imagewise exposing the photoresist to actinic radiation to thereby form image and nonimage areas;
(f) developing the photoresist thereby removing the nonimage areas and leaving the image areas;
(g) etching the foil under the removed nonimage areas of the photoresist to thereby form a copper pattern;
(h) optionally removing the remaining photoresist;
(i) cutting the foil into sections;
(j) optionally punching registration holes through the foil;
(k) optionally treating the copper pattern with a bond enhancing treatment;
(l) optionally inspecting the copper pattern for defects; and
(m) laminating at least one foil section to a substrate; thus forming a multilayered circuit structure.